Implantable stimulation devices are devices that generate and deliver electrical stimuli to body nerves and tissues for the therapy of various biological disorders, such as: pacemakers to treat cardiac arrhythmia; defibrillators to treat cardiac fibrillation; cochlear stimulators to treat deafness; retinal stimulators to treat blindness; muscle stimulators to produce coordinated limb movement; spinal cord stimulators to treat chronic pain; cortical and deep brain stimulators to treat motor and psychological disorders; and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder sublaxation, etc. The present invention may find applicability in all such applications, although the description that follows will generally focus on the use of the invention within a spinal cord stimulation system, such as that disclosed in U.S. Pat. No. 6,516,227 (“the '227 patent”), issued Feb. 4, 2003 in the name of inventors Paul Meadows et al., which is incorporated herein by reference in its entirety.
As an alternative to having a lead or wire pass through the skin of the patient, power and/or data can be supplied to an implanted medical device via an RF or electromagnetic link that couples power from an external (non-implanted) coil to an internal (implanted) coil. So long as a suitable link, e.g., an inductive link, is established between these two coils, which means some sort of external power source must be carried by or worn by the patient, power and/or data can be continuously supplied to the implanted medical device from the worn or carried external device, thereby allowing the implanted medical device to perform its intended function.
It is also known to power an implanted medical device with a battery that is housed internal to the implanted device. However, any battery used for extended periods of time will eventually need to be either recharged or replaced. Replacing an internally implanted battery may subject the patient to further surgery and thus is not desirable, at least not on a frequent basis.
Rather than replace an implanted battery, the battery can be recharged by transcutaneously coupling power from an external source to an implanted receiver that is connected to the battery. Although power can be coupled from an external source at radio frequencies using matching antennas, it is generally more efficient to employ an external transmission coil and an internal receiving coil which are inductively (electromagnetically) coupled to each other to transfer power at lower frequencies. In this approach, the external transmission coil is energized with alternating current (AC), producing a varying magnetic flux that passes through the patient's skin and induces a corresponding AC voltage in the internal receiving coil. The voltage induced in the receiving coil may then be rectified and used to power the implanted device and/or to charge a battery or other charge storage device (e.g., an ultracapacitor), which in turn powers the implanted device. For example, U.S. Pat. No. 4,082,097 discloses a system for charging a rechargeable battery in an implanted human tissue stimulator by means on an external power source.
To allow for flexibility of use and increased comfort to a patient as the implanted battery is charged, the patient would benefit from a convenient unobtrusive external charging device that transmits power transcutaneously to an implanted device, wherein such external charging device is not only small and lightweight, but is also readily conformable to the patient in close proximity to the implanted device. For example, the device could be constructed such that it could be formed to any shape when needed, or the device could be constructed to be shaped in one particular form and then remain in that form for frequent use on the same area of the patient.
In shaping such an external charging device to fit the patient, it is also important to consider the shape of the charging coil in the external charging device. In particular, if the shape of the charging coil in the external charging device changes as the external charging device is shaped to conform to the patient, the characteristics of the charge from the charging coil may change, possibly negatively impacting the coupling factor of the external charging device and the IPG and thus the efficiency of the charging action. Not only does good coupling increase the power transferred from the external charger to the implantable pulse generator, it also minimizes heating in the implantable pulse generator. This in turn reduces the power requirements of the external charger, which reduces heating of the external charger and minimizes the smaller form factor of the external charger. As such, maintaining good coupling may be achieved by monitoring any change in the shape of the coil and subsequently adjusting power requirements of the external charger.
Thus, there remains a need for improved devices and methods for shapeable devices that conform to a surface of the patient while also ensuring that changes in the shape of the charging coil do not negatively impact the charging action of the implanted device.